In recent years, a method of attaching the RFID (Radio Frequency Identification) tag to a package, and performing package management by carrying out wireless communication between the RFID tag and the reader/writer is used. According to such method, the efficiency of distribution task can be achieved since the data such as ID (Identification) can be automatically read out from the RFID tag attached to the package conveyed by a pallet and the like. However, the moving direction of the package and the RFID tag attached thereto cannot be detected, and entering and exit cannot be automatically detected.
A typical known method for detecting the moving direction of the RFID tag is a method using a sensor. In such general method, a sensor for detecting the moving direction of the object is installed at the periphery of the antenna for reading the data from the RFID tag attached to the package. The communication is carried out between the reader/writer and the RFID tag through the antenna when the package attached with the RFID tag is conveyed by humans, machines, and the like, and passes through the front of the antenna. At the same time, the sensor reacts to detect the moving direction of the package, and the person or the machine conveying the same. Therefore, the moving direction of the package, the human, and the machine detected by the sensor can be estimated as the moving direction of the RFID tag.
However, according to the technique using the sensor as described above, the moving direction of the RFID tag is indirectly estimated by reacting the sensor to humans, machines, and the like conveying the package irrespective of the RFID tag. Thus, the sensor sometimes reacts to objects not relevant to the package attached with the RFID tag such as humans, machines, and the like not conveying the package. In this case, the moving direction of the RFID tag cannot be correctly detected since the moving direction of the irrelevant object is estimated as the moving direction of the RFID tag. For instance, if the irrelevant object is moving in the direction opposite to the moving direction of the RFID tag, the moving direction of the RFID tag to be estimated becomes opposite to the actual moving direction.
The technique of specifying the position of the RFID tag is disclosed in patent document 1, but the moving direction of the RFID tag cannot be detected if the RFID tag moves. Furthermore, if another RFID tag moves with such RFID tag, the RFID tags cannot be associated with each other.
Patent document 1: Japanese Laid-Open Patent Publication No. 2006-10345